1. Field of the Invention
The present invention relates generally to semiconductor devices, and more particularly to semiconductor devices having a semiconductor thin film as its active layer and the manufacturing method thereof. The invention also relates to thin film semiconductor transistors with an active layer made of crystalline silicon films.
2. Description of the Prior Art
In the recent years semiconductor thin-film transistor (TFT) devices are becoming more widely used in the manufacture of electronic parts or components, particularly reduced-thickness display devices and digital integrated circuit (IC) packages, as the speed and cost advantages of these devices increase. As such electronics require higher packing density, higher speed, and lower power dissipation, TFTs become more critical in performance and reliability. Some prior known TFTs come with a silicon thin film formed on a substrate with a dielectric surface, which film typically measures several tens to several hundreds nanometers (nm) in thickness.
Typically, the TFT has an active region as defined between spaced-apart source and drain regions for selective formation of a channel region therein. The active region, namely, the channel formation region, as well as its associated source/drain junction regions may play an important role to determine the performance of TFT as a whole. This can be said because the resistance of a current path from the source to drain through the channel, or the mobility of minority charge carriers, can strictly reflect the overall electrical characteristics of TFTs.
Conventionally, amorphous silicon films have been generally employed as the semiconductor thin film constituting the active layer of TFTs. These amorphous silicon films may be fabricated by plasma chemical vapor deposition (CVD) and low pressure thermal CVD techniques.
Unfortunately, the use of such amorphous films is encountered with a problem that where TFTs are required to exhibit higher operation speeds, amorphous films are incapable of trace such trend due to its inherently lowered mobility of charge carriers. To this end, silicon thin films with enhanced crystallinity (to be referred to as the “crystalline silicon film” hereinafter) should be required.
One prior known approach to form such crystalline silicon film on a substrate has been disclosed, for example, in Published Unexamined Japanese Patent Application (PUJPA) No. 6-232059 to be assigned to the present assignee. In this prior art a chosen metallic element is employed to facilitate or accelerate crystal growth of silicon, which is subject to thermal or heat treatment at a temperature of 550° C. for four hours. With this, resultant crystalline silicon film offers enhanced crystallinity. A similar approach has also been disclosed in PUJPA No. 6-244103.
Another prior art approach has been disclosed in PUJPA No. 7-321339, wherein a similar technique is used causing silicon to grow in substantially parallel to the crystal plane of a carrier body, such as a supporting base plate, i.e., substrate. The resulting crystallized region is called the “lateral growth region” in some cases.
The lateral growth region thus formed using the above technique is improved in crystallinity due to the fact that columnar or capillary crystals are gathered with the crystal growth directions being well aligned to one another. The use of such region to form an active layer or layers may contribute to an increase in performance of TFTS.
As the semiconductor manufacturers are commercially demanded to further improve the TFT speed endlessly, even the TFTs with such lateral-growth films as the active layer thereof will be unable to catch up the strict demands due to their inherent limitations as to improvements of the crystallinity.
Advanced active-matrix liquid crystal display (LCD) devices or passive LCDs which employ thin-film transistors (TFTs) for respective picture elements or pixels are examples. The LCDs of these types incorporate peripheral circuitry which includes driver circuits for electrically driving an associative LCD pixel array, image data processor/controllers for handling video signals in a desired format, a memory array for storage of several kinds of information items, and the like. Of those circuit components, the data processor/controllers and memory array are strictly required to be equivalent in performance to presently available advanced integrated circuit (IC) chips as fabricated using single-crystalline wafers. Accordingly, where these LCD driver circuits are integrated on a substrate by use of a semiconductor thin film as formed on the substrate surface, it is required that such thin film exhibit the maximum similarity in nature to the crystallinity of single crystals. Unfortunately, none of the prior art proposed are capable of overcoming this problem. One reason for this is that the lateral growth silicon films do not come without accompanying a problem that reliability and productivity remain lowered due to the fact that the metallic element as used for acceleration of crystal growth might continue to reside within resultant silicon films, which disadvantageously serves to degrade the reproducibility. This is a serious bar to a further advance in semiconductor fabrication technology.